Darwinism 24th Nov, 1859. In his book, Darwin

Darwinism (Theory of Natural Selection):

A. Introduction:

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Charles Darwin (Fig.
7.36) (1809-1882 A.D.), an English naturalist, was the most dominant figure
among the biologists of the 19th century. He made an extensive study of nature
for over 20 years, especially when he joined the expedition on Beagle to South
America. (Fig. 7.37) and explored South America, the Galapagos Islands and
other islands.

He collected the observations on animal distribution and the relationship
between living and extinct animals. He found that existing living forms share
similarities in many ways, not only among themselves but also with the life
forms that existed millions of years ago, some of which have become extinct.

He stated that every population has built in
variations in their characters. He found that all living organisms,including humans
are modified descendants of previously existing life forms. Darwin perceived
the origin of species and adaptations as closely related processes. By the
early 1840’s,Darwin had worked out the major features of his theory of natural
selection as the mechanism of evolution.In 1884,Darwin wrote a long essay on
the origin of species and natural selection,But before it could be published,
Alfred Wallace, a young naturalist,working in East Indies developed a theory of
natural selection, identical to Darwin’s.Wallace’s paper,along with extracts
from Darwin’s unpublished 1884 essay, were presented to the Linnaean society of
London on July 1,1858.                   

Darwin elaborated his theory of evolution in a
book entitled “On the Origin of Species by means of Natural Selection”,
published on 24th Nov, 1859. In his book, Darwin developed the two main points:

·        
Descent with Modification    

·        
 Natural Selection

In this theory, Charles Darwin proposed the
concept of natural selection as the mechanism of evolution.

B. Postulates of Darwinism:

Main postulates of Darwinism
are:

     
Geometric increase.

     
Limited food and space.

     
Struggle for existence.

     
Variations.

     
Natural selection or Survival of the fittest.

     
Inheritance of useful variations.

     
 Speciation.

 

Ø  Geometric Increase:

According to Darwinism, the populations tend
to multiply geometrically and the reproductive powers of living organisms
(biotic potential) are much more than required to maintain their number e.g.,

Paramecium divides thrice by binary fission in
just 24 hours under favourable conditions. At this rate, a Paramecium can
produce a clone of about 280 million Paramecia in just one month and in five
years, can produce Paramecia having mass equal to 10,000 times than the size of
the earth.

Other rapidly multiplying organisms are:
Oyster (114 million eggs in one spawning); Ascaris (70, 00,000 eggs in 24
hours); housefly (120 eggs in one laying and laying eggs six times in a summer
season).

Similarly, the plants also reproduce very
rapidly e.g., a single evening primrose plant produces about 1, 18,000 seeds
and single fern plant produces a few million spores.

Even slow breeding organisms reproduce at a
rate which is much higher than required e.g., an elephant becomes sexually
mature at 30 years of age and during its life span of 90 years, produces only
six offsprings. At this rate, if all elephants survive then a single pair of
elephants can produce about 19 million elephants in 750 years.

These examples confirm that every species can
increase manifold within a few generations and occupy all the available space
on the earth, provided all survive and repeat the process. So the number of a
species will be much more than can be supported on the earth.

Ø  Limited food and space:

According to Darwinism,a population tends to
increase geometrically, but the food increases only arithmetically.
Therefore,the two main limiting factors on the tremendous increase of a
population are: limited food and space, which together constitute the major
part of carrying capacity of environment. These do not allow a population to
grow indefinitely which are nearly stable in size except for seasonal
fluctuation.

Ø   Struggle for existence:

Due to tremendous increase in populations but
limited food and space, there starts an everlasting competition and war between
individuals having similarities in their features. In this competition, every
living organism desires to have an upper hand over others.

This competition between living
organisms for the basic needs of life like food, space, mate etc. for their
survival is called struggle for existence which further is of three types:

(a) Intraspecific:

Between the members of same species e.g. two
dogs struggling for a piece of meat.

(b) Interspecific:

Between the members of different species e.g.
between predator and prey.

(c) Environmental or
Extra specific:

Between living organisms and adverse
environmental factors like heat, cold, drought, flood, earthquakes, light etc.

Out of these three forms of struggle, the
intraspecific struggle is the strongest type of struggle as the needs of the
individuals of same species are most similar e.g., sexual selection in which a
cock with a more beautiful comb and plumage has better chances to win a hen than
a cock with less developed comb.

In this death and life struggle, the majority
of individuals die before reaching the sexual maturity and only a few
individuals survive and reach the reproductive stage. So struggle for existence
acts as an effective check on an ever-increasing population of each species.

The nature appears saying, “They are weighed
in the balance and are found wanting.” So the number of offsprings of each
species remains nearly constant over long period of time.

Ø  Variations:

Variation is the law of nature. According to
this law of nature, no two individuals except identical (monozygotic) twins are
same/identical. This everlasting competition among the organisms has compelled
them to change according to the conditions to utilize the natural resources and
can survive successfully.

Darwin stated that the variations are
generally of two types—continuous variations or  discontinuous variations. On the basis of
their effect on the survival chances of living organisms, the variations may be
neutral, harmful and useful.

Darwin proposed that living organisms adapt  to changing environment due to useful
continuous variations {e.g., increased speed in the prey; increased water
conservation in plants; etc.), as these will have a competitive advantage.

Ø  Natural selection or Survival
of the fittest:

Darwin stated that as humans select the
individuals with desired characters in artificial selection; nature selects
only those individuals out of the population which are with useful continuous
variations and are best adapted to the environment while the less fit or unfit
individuals are rejected by it.

Natural Selection can amplify or diminish only
those variations that are heritable.It is noteworthy to say that adaptations
that an organism acquires by it’s own actions are not heritable.The specifics
of natural selection are regional and timely; environment factors vary from
place to place and from time to time.An example of natural selection in action
is the evolution of antibiotic resistance in bacteria.

 

Ø  Inheritance of useful
variations:

Darwin believed that the selected individuals
with the best features  pass their useful
continuous variations to their offsprings so that they are born fit to the
fluctuated environment.

Ø  Speciation:

According to Darwinism, the unequal ability of
individuals to survive and reproduce will lead to a gradual change in a
population, with favorable characteristics accumulating over the generations
thus leading to the evolution of a new species.

C.
Evidences in favour of Darwinism:

1. There is a close parallelism between
natural selection and artificial selection.

2. The remarkable cases of resemblance e.g.
mimicry and protective colouration can be achieved only by gradual changes
occurring simultaneously both in the model and the mimic.

3. Correlation between position of nectaries
in the flowers and length of the proboscis of the pollinating insect.

D.
Evidences against Darwinism:

Darwinism is not able to
explain:

1. The inheritance of small variations in
those organs which can be of use only when fully formed e.g. wing of a bird.
Such organs will be of no use in incipient or underdeveloped stage.

2. Inheritance of vestigial organs.

3. Inheritance of over-specialised organs e.g.
antlers in deer and tusks in elephants.

4. Presence of neuter flowers and sterility of
hybrids.

5. Did not differentiate between somatic and
germinal variations.

6. He did not explain the causes of the
variations and the mode of transmission of variations.

7. It was also refuted by Mendel’s laws of
inheritance which state that inheritance is particulate.

So this theory explains only the survival of
the fittest but does not explain the arrival of the fittest so Darwin himself
confessed, “natural selection has been main but not the exclusive means of
modification.”

Principle of Natural Selection
(Table 7.7):

It was proposed by Ernst Mayer in 1982. It
stems from five important observations and three inferences as shown in Table
7.7. This principle demonstrates that natural selection is the differential
success in reproduction and enables the organisms to adapt them to their
environment by development of small and useful variations.

 

 

These favourable Variations accumulate over
generation after generation and lead to speciation. So natural selection
operates through interactions between the environment and inherent variability
in the population.